docs: def _get_site_type_dummy_variables(self, sites):
          """
          Get site type dummy variables, three different site classes,
          based on the shear wave velocity intervals in the uppermost 30 m, Vs30,
          according to the NEHRP:
          class A-B: Vs30 > 760 m/s
          class C: Vs30 = 360 − 760 m/s
          class D: Vs30 < 360 m/s
  
          """
          S = np.zeros(len(sites.vs30))
          SS = np.zeros(len(sites.vs30))
  
          # Class C; 180 m/s <= Vs30 <= 360 m/s.
          idx = (sites.vs30 < 360.0)
          SS[idx] = 1.0
          # Class B; 360 m/s <= Vs30 <= 760 m/s. (NEHRP)
          idx = (sites.vs30 >= 360.0) & (sites.vs30 < 760)
          S[idx] = 1.0
  
          return S, SS